In an inactive air conditioning, heat pump, or refrigeration system, pressure equalization takes place and refrigerant tends to condense and collect at cool and/or low locations in the system. For the range of indoor and outdoor temperatures encountered in many systems during the off-portions of their cycles, the compressor is often the coolest part of the system for some period of time. As a result, considerable liquid refrigerant may collect in both suction-side and discharge-side portions of the compressor.
Liquid refrigerant that collects in the compressor oil sump produces a raising of the liquid level but dilutes the oil, reducing its ability to lubricate compressor bearings and other moving parts when the compressor is started. Liquid refrigerant that condenses on the suction side of the compressor may be drawn into the compression mechanism at start-up resulting in a flooded start. Since the liquid is essentially incompressible, its presence can result in very high pressures and stresses in the compressor. Lesser amounts of liquid refrigerant can wash away lubrication oil films normally present on moving parts. Liquid that condenses on the suction side may also be delivered directly or indirectly into the compressor oil sump at start-up, thereby diluting oil with the possible consequences described above.
Because of the affinity between refrigerants and many of the lubricants used therewith, refrigerant may also migrate to, and dissolve into, the oil over time even when the compressor is not any cooler than other portions of the system, thereby contributing to oil dilution and attendant loss of lubricating ability. This affinity also results in oil being removed from the sump and distributed throughout the system by the refrigerant in circulating through the system.
In operation of the system, the greatest heat transfer occurs in the evaporator due to phase change of the refrigerant from liquid to gas. The expansion device controls the flow and pressure drop of the refrigerant entering the evaporator. While superheated refrigerant normally flows from the evaporator to the compressor, if the expansion device does not properly function and/or if insufficient heat is available to achieve complete evaporation of the refrigerant, liquid refrigerant may be supplied to the suction of the compressor. Liquid refrigerant may also be supplied to the compressor if the system is overcharged with refrigerant. Lubrication failure, flooded starts, liquid refrigerant flooding and slugging can each cause compressor failure.